Sauropsida: Class Reptilia 



Fishes live in water. The Class Amphibia straddles the line between 

 water and land. Reptiles are land animals. Some of them manage to 

 live in the water but, even so, they possess the basic anatomic peculiar- 

 ities of land animals. Adaptation of the animal to the external medium 

 in which it lives is most immediately necessary for the skin, the 

 respiratory mechanism, and the locomotor appendages. 



The necessity for adaptation to the environment is not more urgent 

 in any part of the animal than in the skin. The amphibian skin, thin, 

 soft, and coated with a moist mucus, would be wholly impracticable 

 on a big heavy animal living in a dry atmosphere and constantly sub- 

 ject to impact of the body against such solid objects as rough ground, 

 rocks, or bark of trees. The retention of the aquatic type of skin by 

 land amphibians has very narrowly restricted their range of possible 

 life on land. In modern reptiles, the skin is adapted to a land-and-air 

 environment. 



The skins of amphibians and reptiles have the same general 

 structure. In both there is a cellular epidermis and beneath that a 

 dermis composed mainly of fibrous tissue. In the amphibian the outer- 

 most very thin layer of the epidermis is composed of a hard, tough 

 substance like that which constitutes claws, fingernails, and horns in 

 mammals. This horny layer (stratum corneum) is usually onl\ one 

 or two cells in thickness (Fig. 11D). When shed by the animal, as 

 frequently happens, it appears as a gauzy, transparent film, like an 

 unusually thin sheet of cellophane. The reptilian epidermis also pro- 

 duces an external horny layer, but it is indefinitely thicker than that of 

 the amphibian and is constituted of several or many layers of epi- 

 dermal cells (Fig. 357). Its thickness varies with the size of the reptile. 

 This reptilian stratum corneum is hard, very tough, and moderately 

 elastic. It is an ideal protective covering for a land animal. It is me- 

 chanically resistant and impervious to water. It is usually differenti- 



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